Gas turbine engines include a turbine module for extracting energy from a stream of hot, gaseous combustion products that flow through a working medium flowpath. A typical turbine module includes at least one rotor disk with an array of circumferentially distributed blades secured to the disk. Each blade has an airfoil that spans radially across the flowpath. The turbine module also includes one or more arrays of circumferentially distributed vanes axially offset from the blades. Each vane also includes an airfoil that spans radially across the flowpath.
Because the blades and vanes are exposed to the hot combustion gases, it is common practice to make them of temperature tolerant alloys such as nickel or cobalt base alloys. Despite the temperature tolerance of the alloys, it is also common practice to protect the blades and vanes from thermally induced damage. One means of protection is to apply a thermal barrier coating to those blade and vane surfaces, such as the airfoils, that are exposed to the hot gases. Another means is to cool the blades and vanes with a coolant, which is usually relatively cool, pressurized air extracted from an engine compressor. In a cooled blade or vane, the airfoil includes one or more internal cavities that receive the coolant. A multitude of coolant holes penetrates the walls of the airfoil to permit fluid communication from the internal cavity to the flowpath exposed surface. During engine operation, coolant flows into the cavity which then distributes the coolant to the holes. The coolant then flows through the holes to cool the airfoil. The coolant discharged from the holes spreads out to form a thin film of coolant on the airfoil surfaces, thus further protecting the airfoil.
Frequently, engine designers use both thermal barrier coatings and air cooling in combination. During manufacture, the coating is first applied to the substrate-alloy of the blade or vane, and then the coolant holes are installed so that they penetrate through both the coating and the airfoil wall. The holes may be installed by any suitable technique such as laser drilling, electron beam drilling or electro-discharge machining, although the holes are often referred to as “drilled” holes irrespective of the actual technique used to install them. In referring to the holes, we use the term “flowpath side opening” to refer to the hole opening at the flowpath side of the airfoil wall and “cavity side opening” to refer to the hole opening at the airfoil internal cavity.
Despite the efforts to guard against thermal damage, the blades and vanes nevertheless suffer such damage during engine operation and must periodically be repaired or refurbished. A typical repair involves stripping the thermal barrier coating, repairing any damage to the substrate alloy, and subsequently re-applying a replacement thermal barrier coating. This presents a difficulty because the coolant holes are already present in the airfoil walls. If the thermal coating were to be applied over these existing holes, it would then be necessary to install corresponding holes through the newly applied coating. These newly installed holes through the coating would have to be precisely aligned with the existing holes in the substrate alloy. However achieving such alignment is exceedingly difficult. Accordingly, standard practice is to completely plug the existing holes in the substrate with a suitable filler alloy prior to applying the replacement coating. This essentially restores the airfoil walls to their pre-drilled state. The replacement coating can then be applied, and new holes can be installed through both the replacement coating and the restored airfoil walls.
The step of plugging the holes involves introducing a suitable alloy paste into the holes and then thermally processing the blade or vane so that the alloy paste solidifies in the holes. A suitable alloy paste has a metallurgical composition compatible with that of the parent alloy so that it forms a secure bond with the parent alloy and can withstand the rigors of service in an engine. To ensure a high quality repair, it is necessary to completely plug the holes in the substrate. That is, the entire volume of each hole must be completely filled with the alloy paste in order to replicate the pre-drilled state of the airfoil walls. The repair paste does not readily flow into the holes and therefore must be forced into the holes. This is typically done with a paste injection system that includes a reservoir of paste, a hollow injection needle that receives the paste from the reservoir and dispenses it into the holes, and a source of pressurized air for forcing the paste through the needle. A technician inserts the dispensing end of the needle into the flowpath side opening of a hole and operates a foot pedal to apply the air pressure to the paste. The pressurization causes the paste to completely fill the hole until it oozes out the cavity side opening of the hole. The technician then repeats this process for each and every hole to be plugged. After all the holes are filled, the paste is allowed to partially dry and stiffen, which leaves a small spur of semi-hardened paste projecting past the cavity side opening of each hole. The technician then uses a small spatula to reach into the cavity and chip off the spurs. Finally, the blade or vane is thermally processed to solidify the paste and bond the solidified alloy to the parent alloy.
The above described plugging process results in a successful repair, but suffers from certain disadvantages. Because the holes are numerous (one vane repaired by the assignee of this application has more than 450 holes) and because the hole diameters are tiny (on the order of 0.040 inches or about 1 millimeter) the technician's job is repetitive and tedious. The repetitive nature of the process makes it ergonomically suboptimal. The tedious nature of the work can lead to mistakes such as incompletely filled holes or holes that are overlooked entirely. Finally, the work is time consuming both because of the large number of holes and also because of the inspections required to identify any hole filling errors or oversights. Nevertheless, the above described filling technique has been thought to be the only way to reliably fill the entire volume of a typical, small diameter coolant hole.
What is needed is a quicker, more satisfactory way to plug multiple holes.